High purity polycrystalline silicon is widely used as a chemical or an industrial source material in semiconductor devices, solar cells, etc. Also, it is used in manufacturing precision functional devices and small-sized, highly-integrated precision systems. The polycrystalline silicon is prepared by thermal decomposition and/or hydrogen reduction of highly-purified silicon-containing reaction gas, thus causing a continuous silicon deposition on silicon particles.
In commercial-scale production of polycrystalline silicon, a bell-jar type reactor has been mainly used. Polycrystalline silicon products produced using the bell-jar type reactor is rod-shaped and has a diameter of about 50-300 mm. However, the bell-jar type reactor, which consists fundamentally of the electric resistance heating system, cannot be operated continuously due to inevitable limit in extending the maximum rod diameter achievable. This reactor is also known to have serious problems of low deposition efficiency and high electrical energy consumption because of limited silicon surfaces and high heat loss.
To solve these problems, there was developed recently a silicon deposition process using a fluidized bed reactor to produce polycrystalline silicon in the form of particles having a size of about 0.5-3 mm. According to this method, a fluidized bed of silicon particles is formed by the upward flow of gas and the size of the silicon particles increases as the silicon atoms deposit on the particles from the silicon-containing reaction gas supplied to the heated fluidized bed.
As in the conventional bell-jar type reactor, a Si—H—Cl-based silane compound, e.g., monosilane (SiH4), dichlorosilane (SiH2Cl2), trichlorosilane (SiHCl3), silicon tetrachloride (SiCl4) or a mixture thereof is used in the fluidized bed reactor as the silicon-containing reaction gas, which can further comprise at least one gas component selected from hydrogen, nitrogen, argon, helium, etc.
For the silicon deposition to prepare polycrystalline silicon, the reaction temperature, or the temperature of silicon particles, should be maintained at about 600-850° C. for monosilane, while being about 900-1,100° C. for trichlorosilane which is most widely used.
The process of silicon deposition, which is caused by thermal decomposition and/or hydrogen reduction of silicon-containing reaction gas, includes various elementary reactions, and there are complex routes where silicon atoms grow into granular particles depending on the reaction gas. However, regardless of the kind of the elementary reactions and the reaction gas, the operation of the fluidized bed reactor yields polycrystalline silicon product in the form of particles, that is, granules.
Here, smaller silicon particles, i.e., seed crystals become bigger in size due to continuous silicon deposition or the agglomeration of silicon particles, thereby losing fluidity and ultimately being moved downwards. The seed crystals may be prepared or generated in situ in the fluidized bed itself, or supplied into the reactor continuously, periodically or intermittently. Thus prepared bigger particles, i.e., polycrystalline silicon product may be discharged from the lower part of the reactor continuously, periodically or intermittently.
Due to the relatively high surface area of the silicon particles, the fluidized bed reactor system provides a higher reaction yield than that by the bell-jar type reactor system. Further, the granular product may be directly used without further processing for the follow-up processes such as single crystal growth, crystal block production, surface treatment and modification, preparation of chemical material for reaction or separation, or shaped body or pulverization of silicon particles. Although these follow-up processes have been operated in a batchwise manner, the manufacture of the granular polycrystalline silicon allows the processes to be performed in a semi-continuous or continuous manner.
The biggest stumbling block in continuous production of particle-shaped, i.e., granular polycrystalline silicon using a fluidized bed reactor is that silicon deposition by the reaction gas occurs not only on the surfaces of the silicon particles heated to a high temperature but also on the surfaces of the reactor components that are inevitably exposed to or in contact with the hot silicon particles.
Silicon deposition occurs and is accumulated on the hot solid surfaces inside the fluidized bed reactor, including the silicon particles, the inner wall of the reactor tube and the reaction gas supplying means, all of which are exposed to the reaction gas. The thickness of the accumulated deposition layer increases with time. Here, it beneficially conforms to the purpose of the fluidized-bed deposition process that the thickness of the silicon deposition layer gradually increases on the surfaces of the silicon seed crystals or silicon particles. It is however disastrous when silicon deposition exceeds an allowable level at the solid surfaces of the reactor components, except for the silicon particles, including the inner wall of the reactor tube and/or the reaction gas supplying means exposed to or in contact with the high-temperature fluidizing silicon particles. If silicon deposition on such reactor components exceeds the allowed extent of mass or thickness, they should be greatly deteriorated in mechanical stability and, in the long run, the operation of the reactor has to be stopped.
For economical production of particle-shaped, i.e., granular polycrystalline silicon, improvement of the productivity of the fluidized bed reactor is essential. Further, for continuous operation of the fluidized bed reactor, which is the primary advantage of the fluidized-bed silicon deposition process, physical stability of the reactor components should be secured most of all. Thus, in order to improve the productivity of the fluidized bed reactor and secure the mechanical stability of the reactor during the silicon deposition process for preparing polycrystalline silicon particles, it is required to effectively remove the silicon deposit which is formed at the reactor components due to their constant exposure to and contact with the hot silicon particles and the reaction gas. Such an effective removal of the silicon deposit is more important in bulk production of polycrystalline silicon particles using a fluidized bed reactor. But, there are only a few techniques related to this.
U.S. Pat. No. 5,358,603 (1994) discloses a method for removing by an etching method the silicon deposit formed specifically on the product discharging means of the fluidized bed reactor during the fluidized-bed silicon deposition process. This method using an etching gas may also be applied to the removal of the silicon deposit formed at the inner wall of the reactor tube. However, application of this method basically requires following steps: first the deposition operation should be stopped; all the silicon particles within the fluidized bed should be discharged out of the reactor; then a heating means should be inserted into the reactor to heat up the silicon deposit for an etching reaction, etc. Such cumbersome and time-consuming steps limit the application of this method to the fluidized-bed deposition process.
U.S. Pat. No. 6,541,377 (2003) discloses a method of preventing silicon deposition on the outlet surface of the reaction gas supplying means or removing the silicon deposit formed thereon during deposition operation, wherein such objects are achieved by supplying an etching gas including hydrogen chloride without interfering with the supply of the reaction gas. This method can solve the problem of silicon deposition at the outlet of the reaction gas supplying means without affecting the operation of the silicon deposition process. However, since the etching gas is selectively supplied near the outlet of the reaction gas supplying means, the method cannot be applied for removing the silicon deposit formed and accumulated on the inner wall of the reactor tube.
U.S. Pat. Nos. 4,900,411 (1990) and 4,786,477 (1988) disclose a method of preventing silicon deposit from accumulating at the gas supplying means and at the inner wall of the reactor tube by circulating a cooling fluid around the corresponding components. However, since an excessive cooling of such reactor components in continuous contact with hot silicon particles consumes unnecessarily a huge amount of energy, this method is economically unfavorable considering an additional facility investment and a high production cost due to heavy energy consumption for heating silicon particles compensating the energy loss.
Different from other materials used in general chemical processes, the components of the fluidized bed reactor for preparing high purity polycrystalline silicon, especially, the reactor tube in contact with the silicon particles should be employed such that an impurity contamination of them should be avoided to the highest degree possible. Therefore, selection of the material for the reactor tube is much restricted. Due to the high reaction temperature and the characteristic of the reaction gas, metallic materials cannot be used for the reactor tube. Meanwhile, it is very difficult, in practice, to find a non-metallic, inorganic material that can prevent the impurity contamination of the silicon particles and ensure sufficient mechanical stability even when the silicon deposit becomes heavily accumulated at its inner wall.
The reactor tube of the fluidized bed reactor for preparation of polycrystalline silicon, which is in incessant contact with hot, fluidizing silicon particles, is vulnerable to irregular vibration and severe stress. Thus, it is very dangerous to continue silicon deposition if the thickness of the silicon deposit on the inner wall of the reactor tube exceeds an allowed value.
When removing by a chemical reaction or an etching reaction the silicon deposit formed and accumulated on the inner wall of the reactor tube during the silicon deposition for preparation of silicon particles, a large portion of the silicon particles fluidizing inside the reactor tube can also be removed together. That is, selective removal of the silicon deposit is almost impossible. Thus, it is the common practice to stop the silicon deposition, cool the inside of the reactor while purging with such an inert gas as hydrogen, nitrogen, argon, helium or a mixture thereof, discharge or withdraw the cooled silicon particles out of the reactor, disassemble the reactor and replace the reactor tube with a new one, reassemble the reactor, fill silicon particles into the reactor tube, heat the silicon particles sufficiently and supply the reaction gas again to resume the preparation of silicon particles. However, much is needed for the disassembling and reassembling of the fluidized bed reactor. In addition, the reactor tube tends to break when the reactor is cooled because of the difference in the degree of thermal expansion of the silicon deposit and the reactor tube material. Consequently, the silicon particles remaining inside the reactor tube are contaminated and the fragments of the reactor tube make the process of disassembling difficult.
Because the silicon deposit accumulated on the inner wall of the reactor tube reduces the productivity of the fluidized bed reactor and increases the production cost, a solution for this problem is needed.